Just moved over from Simple Press and I can't believe how many spam members I have after only two days.

How can I stop this?

Welcome to the club. There's a lot of spamming going on. Take this for what it's worth but when I started using this message I conjured up myself - my spammers all but stopped -

You can't post ANYTHING or use your account until an Admin approves your account based on spam databases and heuristic screening criteria - you will not be registered until this approval is complete - if you still wish to apply enter "notspammer" without the quotes in the box

Place a question (Admin > Security and Moderation > Anti-Spam.) that is in direct relationship to your content of your forums. something your users would know. This will throw the bots off the tracks for sure.